It's not an exact science but it's a known fact that there are times when your vocal ability is not going to be as good as others. This can be due to all sorts of things; not feeling well, not properly warmed up, drinking, eating or ingesting something that affects your vocal chords, tiredness, stress, not enough practice, too much practice. You get the idea.
The one constant in this is that it's also a known fact that the more you do it, the better you become. This is noticeable and prevalent in music (and many of the other arts), than just about any other thing. Although it has been studied and proven in areas such as driving cars to a certain age (although this correlates with other motor skills like instrument playing).
One of the great things about music is that it seems for most people, the older they get and the more time they spend with their craft, the better they get.
I guess this is just a long winded way of agreeing with Lynn, there is no substitute for practice, and practice makes perfect, and record everything.